


So This Is How It Starts

by pandorium



Series: You've Tattooed Sin Into My Heart [1]
Category: Supernatural
Genre: Alternate Universe, Alternate Universe - Canon Divergence, Alternate Universe - High School, Always a girl sam, Angst, Angst and Fluff and Smut, F/M, Fluff, Genderswap, High School, Lust, Sibling Love, Smut, Teen Romance, Teenagers, True Love, girl!Sam
Language: English
Status: Completed
Published: 2020-03-31
Updated: 2020-03-31
Packaged: 2021-02-23 05:29:25
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings, Underage
Chapters: 2
Words: 1,511
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23406487
Author URL: https://archiveofourown.org/users/pandorium/pseuds/pandorium
Summary: Dean comes to realize Sam is more than just a little sister.
Relationships: Dean Winchester/Sam Winchester
Series: You've Tattooed Sin Into My Heart [1]
Series URL: https://archiveofourown.org/series/1683739
Comments: 6
Kudos: 60





	1. Chapter 1

**Author's Note:**

> The title is based off of a 1975 song called "Sex". If I continue the series it won't be the only one either.

It happened when he was driving Sam to school before his shift at the garage started. The sun was shining uncharacteristically bright, even for the middle of August, catching on the paleness of her thighs where the skirt of her dress rode up. He’d never noticed how creamy they were before, never really wondered if they were as smooth as they looked. Hadn’t thought about them past how long they were becoming and how much he could tease her about it until it got old. 

And yet he found himself sneaking furtive glances at her, eyes trailing up from her once knobby knee, over the curve of her hip, and up to the front of her dress where the soft rounds of her breasts crested above the neckline each time she took a breath like it was too tight. His mouth suddenly felt dry, tongue thick and heavy as he struggled to swallow. He and Dad would have to scrape together some money to buy her some new clothes. Ones that weren’t so…just plain fucking indecent on her.

Dean wasn’t sure when Sammy had grown up, but suddenly his little sister wasn’t so little anymore and it made his throat tighten in disgust that he’d noticed almost as much as the sight of her sent a sharp spark of arousal zinging low in his gut.

He forced himself to keep his eyes on the road for the rest of the ride, shameful heat flushing his cheeks. But Sam was fidgeting next to him, nerves at starting the new school year palpable and thickening the tension created by his startling new discovery that Sammy wasn’t just his kid sister. He longed to settle her nerves, fingers twitching with the urge to settle on her thigh and curl right above her knee like they had so many times before. The comforting gesture, so familiar and automatic seemed so wrong, like that one touch would be improper now that he’d realized his sister’s scrawny legs have transformed into long, lean stretches of muscle beneath all that soft skin.

An eternity stretched between his sudden awareness of Sam’s body and the moment she was pecking him on the cheek in thanks before bouncing out of the car to join the rest of her friends on the front lawn. Dean jerked out of his reverie and grinned as Sam’s friends waved ‘hello’ to him before he pulled away. The familiar burn of satisfaction at the way they all immediately huddled together to giggle and gossip about him flushed through his chest and he pushed the thought of his sister’s too tight clothing and foal-like legs from his mind.

****

The second time, the air conditioner had broken, forcing their family to exist in a state of perpetual semi nudity just to stay sane, and Dean was going out of his mind. Sam had been parading around in less and less. Her endless expanse of pale, unmarred skin was slowly dusted a light gold color from exposure to the sun’s harsh rays. It was another unfortunate temptation aided and, in fact, caused by spending all of her time by the pool in an effort to stay cool. 

If he wasn’t such an apparent masochist, he’d hole himself up in his room to suffer through the heat instead of swimming laps and lounging on the pool floaties while Sam splashed around him.

Because he really shouldn’t be noticing that the last vestiges of baby fat clinging to her body were recently replaced with gentle sloping curves, ones that would no doubt fill out the older she got. Nor should he notice the sharpness of her collarbones or the smooth column of her throat just begging to be kissed and nipped until she was covered in marks.

Every glance, every prolonged second he looked at her, kept guilt constantly shredding away at his insides. It carved itself a nest, burrowing into his chest, until the air con gods finally took pity on him and theirs was fixed. (More like their father “found” one that had “fallen off the back of a truck”, but semantics.) Immediately he locked himself in his bedroom with multiple copies of Busty Asian Beauties and avoided Sam and their Dad for days until thoughts of his sister were banished from his mind.

****

  
The third time they were sparring, their Dad away on another hunting trip, gone for the week at least. Sam’s stomach was flat and toned, tan and smooth, shiny with a thin sheen of sweat and Dean couldn’t help but look. To watch the way her muscles bunched and quivered as they danced around each other for a moment before engaging in combat once more.

It wasn’t until he was pinning her to the ground, hands perched over her ribs, fingers spread across the slats of bone that Dean had the urge to hurl though. He was hard. She couldn’t tell, she was squirming against his hold and making matters worse. 

It wasn’t the first time an awkward boner had occurred, but it was the first time that it wasn’t solely from the wind blowing just the right way and Dean could hardly breathe as he pushed himself up roughly, abruptly declaring the session over.

****

The fourth time he caught himself staring, so did she. And those tempting red lips his eyes had been fixed upon, _**smirked**_.


	2. Chapter 2

**Summary for the Chapter:**

> Sam catches Dean noticing.

The first time she caught him she burned red, and her fingers drummed out a beat against the thigh that seemed to have captured his attention. The one bouncing with anxiety, heel beating out a rhythm against Baby’s floor boards. Sam wasn’t even sure he knew he was staring, knew that his gaze was powerful enough to garner a bodily response and light her nerves on fire. She pressed her mouth in a thin line and resolutely ignored it, changing her mind a moment later and biting at her bottom lip as she waited for her school to come into view.

Sam could feel her chest heaving, could feel the fabric of her dress pulling at her as her rib cage expanded. It was too small but it was the only thing close to acceptable in her closet after her recent growth spurt. She didn’t want to mention anything to Dad, they had just made a trip to the Goodwill not too long ago and she knew he’d be pissed at having to go again so soon.

She tugged at the hem of her dress, and nearly vibrated out of her skin before she was kissing Dean on the cheek and hopping out of the car, frantically fleeing to the safety of her friends. 

****

The air conditioner was broken and Sam couldn’t stand the heat. It made her cranky and sticky and just all around pissy. She couldn’t help but be grateful for the too small clothing she still had in her pack when she’s able to dig out shorts so short they could be a belt and a tank top that was barely thick enough to remain opaque. In them she wandered the house for days, forgoing all other clothing until she was forced to change to wash the outfit.

But it wasn’t until she was lounging by the pool that she felt his eyes for the second time, sweeping over her prone form with a brooding intensity. Protected by her sunglasses Sam found herself looking back, eyes catching on the strong expanse of Dean’s back and the broad heft of his shoulders as he glanced back over his shoulder at her and his gaze lingered a bit too long.

****

They were sparring and her skin was on fire, stretching and pulling over tight muscles as she fought her older brother. He had at least eighty pounds on her and almost four more years of experience. But it wasn’t about winning. It was about holding out as long as possible before he pinned her. Except the way that he stared had her gasping and squirming, jittery and unfocused as she took jab after jab at him until her feet were swept out from under her and she was on her back once more.

Her head thunked back on the ground, eyes closing against the persistence of the sun, body unable to rest beneath Dean’s weight. She struggled enough to put on a show before her brother was pushing off her, the rough press of his hands stealing her breath away. Jerk.

****

The fourth time she was ready. Dean may have gotten the perfect mouth in the family but Sam knew she was no slouch either. She was a Winchester after all and she had a natural pout that most girls would kill for. So, she blotted her lips after a pass with cherry red lipstick she’d borrowed from one of her friends-John would never allow any money to be wasted on makeup-and waited until she knew she had his attention. The corners of her lips curled and her tongue darted out to flick over the fullness of her bottom lip just before she fucking smirked.

**Author's Note:**

> So you may have seen this already. It's just reposted with a few tweaks. I think I'm going to make this a series if I can remain motivated. Comments and kudos are appreciated and thank you to all who read this!


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